Japanese Laid-Open Patent Publication No. 178730/1982 discloses a container closure comprised of a metallic container closure body and a plastic grip ring as a suitable example of an easily openable container closure. The container closure body is made of a suitable thin metallic plate such as a thin aluminum-base alloy plate and has a circular top panel wall, a cylindrical skirt wall extending from the peripheral edge of the circular top panel wall, and a linking protruding piece protruding from part of the free end of the skirt wall. The plastic grip ring is integrally linked to the linking protruding piece of the container closure body.
Furthermore, Japanese Laid-Open Patent Publication No. 302010/1988 discloses a compression molding apparatus which compression-molds a plastic grip ring of a container closure of the above type and simultaneously links it integrally to a linking protruding piece of a container closure body. The above compression molding apparatus is equipped with a compression molding means that includes a rotary support member which is rotatably mounted, and a plurality of molds mounted on the rotary support member maintaining a distance in the circumferential direction. As the rotary support member rotates, the molds are successively conveyed via a plastic material feed zone, a container closure body feed zone, a compression molding zone and a discharge zone. The compression molding apparatus is further equipped with a plastic material feed means for feeding a heated and molten plastic material to each of the molds in the plastic material feed zone. The plastic material feed means is constituted by an extruding block having a plastic material flow path formed extending from a receiving port to a discharge port, and an extruder coupled to the extruding block. The discharge port of the extruding block is positioned in the plastic material feed zone, and the receiving port of the extruding block is communicated with an extruding port of the extruder. The heated and molten plastic material extruded from the extruder is discharged from the discharge port through the plastic material flow path in the extruding block, and is fed to the mold.
The conventional compression molding apparatus involves, however, the following problems that must be solved. The operation of the extruder is stabilized only after some periods of time (e.g., two to three minutes) have passed from the start of its operation in the plastic material feed means, and hence, the plastic material extruded from the extruder is not fully adapted to being compression-molded until the above periods of time have passed. Unacceptable products are produced if the compression molding is executed using such an unsuitable resin material. In the compression molding apparatus of the type mentioned above, furthermore, if the grip ring is compression-molded by feeding the plastic material under the condition where the container closure body has not been fed to the mold in the compression molding means, then the grip ring formed might be not discharged from the mold but remain therein. That is unacceptable situation. It is therefore desired that the plastic material extruded from the extruder is fed to the mold or is prevented from being fed to the mold in accordance with the necessity. However, the conventional compression molding apparatus could not satisfy such requirements. It is desired that the extruder is continuously operated from the standpoint of stability in its operation. Therefore, it is not allowed to control the feeding of plastic material by occasionally stopping the operation of the extruder.